todo.t: better check for DEBUGGING

cppflags isn't universal and may or may not be the mechanism via
which the C macro DEBUGGING gets defined.  So use $Config{config_args},
which seems to have worked in several similar cases in the past.

Author: craigberry

t/run/todo.t

@@ -103,7 +103,7 @@ use warnings;
 
 my $switches = "";
 
-my $is_debugging_build = $Config{cppflags} =~ /-DDEBUGGING/;
+my $is_debugging_build = $Config{config_args} =~ /\bDDEBUGGING\b/;
 
 our $TODO;